Mars is so small because Jupiter shook up its formation

Mars can blame Jupiter for its small stature. The Red Planet may be much smaller than we expect because Jupiter’s gravity beat it up as it was forming.

Models of our solar system’s formation suggest that Mars should be between 1.5 and two times Earth’s mass. Instead, it weighs in at a mere one-tenth the mass of our world.

Now an old theory that might explain why is resurfacing: gas left over from the formation of Jupiter meddled with the rocks that ultimately built Mars, making them fall apart rather than clump together.

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The gas giants formed by accreting gas from the protoplanetary disc that surrounded the sun. As they grew, their gravity started to have more impact than the remaining disc on the still-forming rocky planets.

The disc’s gravity pulled the protoplanets’ axes of rotation in one direction, but the gravity from Jupiter came from the opposite direction, tugging them that way. When those competing forces balanced in a certain way, the protoplanets felt a kick from Jupiter’s gravity at the same point in their orbit around the sun, an effect known as sweeping resonance.

“Before the kicks, collisions between solids occurred at low velocity, so they merged,” says Scott Kenyon at the Harvard-Smithsonian Center for Astrophysics. “After the kicks, the collisions are at high velocity, so colliding objects fragment.”

Revived theory

A trio of researchers led by Douglas Lin at the University of California at Santa Cruz first proposed this general scenario over a decade ago. Now Kenyon and Ben Bromley at the University of Utah have revived it specifically to explain Mars’s diminutive size.

“Recent studies of meteorites suggest Mars formed much more rapidly than [Lin’s team] assumed, so we worked out the consequences for a disc that evolves more rapidly,” Kenyon says.

Lin and his colleagues also published a paper in February in TheAstrophysical Journal suggesting Jupiter’s sweeping resonance could have been responsible for clearing the asteroid belt of rocks smaller than 50 kilometres wide.

The theory might also hold for other solar systems, suggesting asteroid belts could be common wherever there are massive planets. “It is a rich subject,” Lin says.